Drawing Out the Facts

The STEM Chicksmas Day Seven: Neutrino Oscillations

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

What’s your favorite flavor of neutrino? Neutrinos belong to a category of fundamental particles called leptons. (To read more about particles and their role in the Standard Model of physics, check out our post on the Higgs Boson.) There are three types, or in physicist parlance “flavors” of leptons, the electron, muon, and tau particles. These flavors can be found in both neutral and charged particles—the charged particles are simply named after the flavor (e.g. electron) and the neutral particles are neutrinos (e.g. electron neutrinos).

Leptons only interact with three of the four fundamental forces: the weak force, gravity, and the electromagnetic force. Neutrinos, being neutral, do not interact with the electromagnetic force. Furthermore, the weak force is, as the name implies, weak and a short-range interaction, and gravity has a very weak effect at the subatomic level. Neutrinos thus pass through matter as if it weren’t there. Detecting them, even now, is not an easy task. So when scientists in the 60s noticed that they only detected one third of the total number of neutrinos they expected to come from the sun, they figured that there was a problem with their model or their detection techniques. However, this one third remained consistent. No research group could account for the missing two thirds empirically.

Two sources of neutrinos are solar fusion and cosmic radiation. Muon neutrinos, for example, come from cosmic radiation, whereas solar fusion only produces electron neutrinos. Some physicists speculated that neutrinos traveling from the sun “changed flavor” on their way to Earth, which is why we could not detect them. (The detection methods were usually specific to electron neutrinos.) However, this would only be possible if the neutrinos had mass, and according to the Standard Model they were massless.

Why? Something important to keep in mind is that in quantum physics, particles are also waves—this is called wave-particle duality. Light is the best known example—it is simultaneously a wave and a photon. Waves are also capable of interfering with each other. When two wavelengths travel at different frequencies, they go out of phase with each other. When waves are out of phase, the amplitudes and shape of the superimposed wave is like an average of the composite waves. This is relevant for neutrinos because neutrinos also oscillate. Because the three flavors of neutrons have slightly different masses, they also move through space at different rates and frequencies. The case for neutrinos is more complicated, because both mass and flavor (specifically the interaction with their charged counterpoints) affect their quantum mechanical properties. The result is, however, that as neutrinos move through space, they oscillate and change flavor. If you look at our graphic, you see that the peaks of the different neutrino flavors shift in relation to each other as they move across the x-axis. This happens only if neutrinos have mass.

2015 Nobel Laureates in Physics Takaaki Kajita and Arthur B. McDonald were able to prove that neutrinos have mass. Kajita and McDonald worked at two different detectors. At the Super-Kamiokande, neutrinos passed through a tank of very pure water that was deep within the ground. When muon and electron neutrinos passed through the water, they would sometimes collide with an atom nucleus or an electron in the water. When they did this, they would make charged particles (muons and electrons). The charged particles would move faster than the speed of light in water (75% of the speed of light in vacuum), and generate a blue light called the Cherenkov light. This light could be measured by detectors in the tank and the neutrino source determined. One curious aspect was that there were way more muon-neutrinos coming from the atmosphere straight above the detector than from those that traveled through the earth, ie those that traveled a further distance. This was explained at the Sudbury National Observatory. They ran a similar experiment on electron neutrinos from solar radiation, and using heavy water (water with deuterium instead of hydrogen), they detected the full number of neutrinos suggested by the Standard Model in all three flavors, not just electron-neutrinos.

The discovery at the Super-Kamiokande suggested that the distance traveled was related to the flavor of neutrino expressed. This makes sense when considering the probability of neutrinos’ waves going out of phase. The discovery at the Sudbury National Observatory accounted for the infamous missing two thirds of neutrinos. The fact that neutrinos have mass is important because it violates certain ideas set forth by the Standard Model, such as lepton conservation. Furthermore, it is important because we still don’t fully understand the role neutrinos play in the universe. There are billions and billions of neutrinos, and we don’t really know what they do. Some theorize that neutrinos play a role with hot dark matter. However, discovering neutrinos have mass is one step forward towards fully understanding particles and the universe.

Read more at the Nobel Prize website or check out the award winning work here (Kajita) or here (McDonald).